1. Field of the Invention
The present invention relates to a movable head position controlling device which is used for magnetic recording and reproducing apparatuses such as a video tape recorder (hereinunder referred to as "VTR").
2. Description of the Related Art
In a magnetic recording and reproducing apparatus such as a VTR, a movable head is used. The movable head is a head which is displaced in accordance with a deflection signal which is supplied from a driving control device. In a VTR, the movable head is provided in such a manner that the end of the movable head projects from the peripheral surface of a rotary drum. By driving the movable head in the state in which a magnetic tape passes over the peripheral surface of the rotary drum, the magnetic head is enabled, for example, to follow a track which is formed on the magnetic tape.
FIG. 46 shows an example of the structure of a movable head. The movable head is composed of a tongue-like piezoelectric bimorph 101 which is bent in accordance with the voltage applied thereto and a magnetic head 103 which is disposed at the free end of the bimorph 101. The magnetic head 103 is fixed to the bimorph 101 and the other end of the bimorph 103 is fixed to the inside of a rotary head (not shown). FIG. 46, and later-described FIGS. 49 and 50 show the positions of a bimorph and a magnetic head on a rotary drum. In these drawings, the circle at the back of the bimorph and the magnetic head represents the outline of the rotary drum.
In FIG. 46, a sensor 102 is a piezoelectric generator formed by cutting a part of the bimorph 101.
When a voltage is applied to the bimorph 101 and the bimorph 101 is warped in accordance with the voltage, the bimorph 101 assumes the state shown in FIG. 47. The angle .theta. (deg) shown in FIG. 47 is called the amount of inclination of the magnetic head 103 and .xi. is called the displacement of the magnetic head 103.
FIG. 48 shows the relationship between the amount of inclination .theta. (deg) of the magnetic head 103 and the effective length of the bimorph 101. As shown in FIG. 48, as the effective length of the bimorph 101 becomes longer, the amount of inclination of the magnetic head 103 reduces. As the amount of inclination of the magnetic head 103 becomes smaller, the end surface of the magnetic head comes into better contact with the magnetic tape which passes over the peripheral surface of the rotary drum, so that it is possible to record or reproduce a high-frequency signal with higher accuracy. Especially, in the case in which it is necessary to greatly displace the magnetic head 103, that is, it is necessary to increase .zeta. as at the time of superior reproduction of a VTR, it is preferable that the amount .theta. (deg) of inclination of the magnetic head 112 is small, that is, the bimorph 101 has a long effective length.
The structure of a bimorph having a longer effective length is shown in FIG. 49. In FIG. 49, a bimorph 201 is comparatively long and it is disposed obliquely relative to the radius of the rotary head. The effective length (the projected length in the right and left direction in the drawing) of the bimorph 201 is therefore longer than that of the bimorph 101 shown in FIG. 46.
The structure of a bimorph having an even longer effective length is shown in FIG. 50. In FIG. 50, an annular bimorph 301 is adopted. The effective length (the projected length in the right and left direction in the drawing) of the bimorph 301 is therefore longer than that of the bimorph 201 shown in FIG. 49.
On the other hand, if the effective length of a bimorph is long, the serial resonance frequency and the parallel resonance frequency become low. FIG. 51 shows an example of the frequency characteristic of a bimorph. In a general frequency characteristic of a bimorph, the phase reverses by 180.degree. when the frequency reaches the primary serial resonance frequency (the frequency at the lower serial resonance point of the two shown in FIG. 51). For this reason, in a VTR system in which tracking is carried out by a movable head, the control frequency band is set at a frequency band lower than the primary serial resonance frequency. If the interval between the primary serial resonance frequency and the secondary serial resonance frequency or the interval between the primary serial resonance frequency and the parallel resonance frequency is long, the control frequency band is set at a frequency band between the primary serial resonance frequency and the secondary serial resonance frequency or between the primary serial resonance frequency and the parallel resonance frequency by phase advancing control. Accordingly, when the effective length of a bimorph is long, it is difficult to secure a sufficiently wide control frequency band, thereby making sufficient tracking difficult. For example, if a track formed on the magnetic tape is rolled, it is difficult for the magnetic head to follow the rolling.
Such a defect can be ameliorated by reducing the peak gain at the serial resonance frequency to a certain extent. By differentiating the output of the sensor, it is possible to reduce the peak of the gain at the serial resonance frequency which is contained in the output.
It is also possible to use an actuator having the structure shown in FIG. 52 in order to drive the magnetic head. The structure of the actuator shown in FIG. 52 will now be explained and the structures of conventional control devices will next be explained.
The actuator shown in FIG. 52 displaces a magnetic head at a large amplitude without inclining the magnetic head. Two magnets 401 and 402 are vertically provided in the actuator, and a yoke 403 is disposed between the magnets 401, 402. An annular yoke 404 is further provided with a gap between the annular yoke 404 and the yoke 403. On the upper side and the lower side of the yoke 404 are provided yokes 405 and 406, respectively. These yokes form a magnetic path for the magnetic fluxes produced by the magnet 401 or 402.
In this actuator, the magnet 401, the yoke 403 and the magnet 402 are disposed within a bobbin 408 which has an actuator coil 407 wound therearound. The actuator coil 407 is situated between the yokes 403 and 404, namely, within the gap. Therefore, when a current flows on the coil 407, a force is applied to the coil 407 in the vertical direction in the drawing. The direction in which the bobbin 408 is moved by this force is regulated by gimbal springs 409 and 410, and the bobbin 408 vertically moves.
Each of the gimbal springs 409, 410 is composed of a discal thin metal sheet provided with a plurality of arcuate slits (not shown). At the centers of the gimbal springs 409, 410, holes are formed, and the bobbin 408 is fitted in the holes so as to be fixed on the inner peripheral edges of the gimbal springs 409, 410. The outer peripheral edges of the gimbal springs 409, 410 are fixed to the yokes. The gimbal springs 409, 410 are parallel to each other.
One 410 of the gimbal springs is integrally formed with a leaf spring 411. The leaf spring 411 is fixed to the bobbin 408 and the free end thereof is fixed to a magnetic head 412. Therefore, the actuator shown in FIG. 52 can displace the magnetic head 412 at a large amplitude without inclining the magnetic head 412.